Separation of components of vaporous fluids

ABSTRACT

High molecular weight components are separated from lower molecular weight components of a vaporous fluid stream. A first vaporous fluid feedstock is expanded. A second vaporous fluid feedstock is compressed. Said expanded and compressed streams are then passed in indirect heat exchange relationship to cool said compressed stream. The cooled compressed stream is then passed to a separation zone to separate the more volatile lower molecular weight components from the less volatile higher molecular weight components.

1 Jan. 15, 1974 SEPARATION OF COMPONENTS OF VAPOROUS FLUIDS [75]Inventors: Walter C. Hart, Bartlesville; Suman P. N. Singh, Stillwater,both of Okla.

[73] Assignee: Phillips Petroleum Company,

Bartlesville, Okla.

22 Filed: Mar. 30, 1970 21 Appl. No.: 23,940

[52] US. Cl 62/23, 62/38, 62/26 [51] Int. Cl F25j 3/06 [58] Field ofSearch 62/23, 24, 26, 27, 62/28, 30, 38, 39, 41

[56] References Cited UNITED STATES PATENTS 2,134,700 11/1938 Brewster62/39 2,134,702 11/1938 Brewster 62/39 2,502,250 ,3/1950 Dennis 62/392,666,019 1/1954 Winn 62/39 2,970,451 2/1961 Ehrlich. 62/23 3,119,6771/1964 Moon 62/28 3,261,168 7/1966 Ruhemann 62/38 3,433,026 3/1969Swearingen.... 62/39 2,503,939 4/1950 DeBaufre... 62/38 2,530,60211/1950 Dennis 62/41 2,713,781 7/1955 Williams.... 62/39 3,205,6699/1965 Grossman 62/38 Primary Examiner-Norman Yudkotf Att0rneyYoung &Quigg [57] ABSTRACT High molecular weight components are separated fromlower molecular weight components of a vaporous fluid stream. A firstvaporous fluid feedstock is expanded. A second vaporous fluid feedstockis compressed. Said expanded and compressed streams are then passed inindirect heat exchange relationship to cool said compressed stream. Thecooled compressed stream is then passed to a separation zone to separatethe more volatile lower molecular weight components from the lessvolatile higher molecular weight components.

9 Claims, 2 Drawing Figures PATENTEB JAN 1 51974 SHEEI 1 BF 2 u S 5 Fl 5d v v H. w w u m 6 mm o N N w@ 9 Q I 1 VA A mm MR mm 9 n mOP mO ImQINVENTORS w. c. HART s, P. .N. SING BY H ATTORNEYS 1 SEPARATION OFCOMPONENTS OF VAPOROUS FLUIDS This invention relates to the separationof higher molecular weight components from lower molecular weightcomponents of a vaporous fluid stream.

It is known in the prior art that higher molecular weight components canbe separated from lower molecular weight components of a vaporous fluidfeed stream by subjecting the feed stream to cryogenic temperaturesusing various refrigerants (such as propane, ethylene, nitrogen, etc.).In these processes, the feedstock is brought to a sufficiently lowtemperature under pressure thereby liquefying the entire volume of thevaporous fluid feed. Subsequently, the lower molecular weight componentscan be flashed from this liquefied mixture.

The above-mentioned and other prior art methods of separating valuablehigher molecular weight components from a vaporous fluid feed streamhave several disadvantages. For example, the well known cascadetyperefrigeration systems require a heavy outlay of equipment which leads tohigh initial plant investment and high operating costs. Additionally,the cascadetype system requires facilities to purify, replenish, andstore the refrigerants such as propane.

Therefore, there has been a need in the art for an efficient process forthe separation of the higher molecular weight components from lowermolecular weight components of a vaporous fluid feed stream employing aminimum amount of equipment. In particular, a process has been neededwherein ethane and other heavier components can be separated fromnatural gas. This need is aggravated by the fact that the separation ofthe ethane and other heavy components from the natural gas mustsometimes be performed in areas which are relatively remote with regardto availability of power supply and other natural resources. Theutilization of the ethane and heavier components as petrochemicalfeedstocks has created a need for new systems of low initial plantinvestment which can effect the separation of these valuable compoundsat economical operating costs. Moreover, it is sometimes desirable tohave a system which does not use a large volume of an externalrefrigerant, such as propane, etc., because this eliminates the highcost of handling these refrigerants.

The present invention provides a solution to the above problems. We havenow discovered that higher molecular weight components can beeconomically separated from lower molecular weight components of avaporous fluid feed stream by a process wherein a first vaporous fluidfeed stream is expanded, a second vaporous fluid feed stream iscompressed, said expanded and compressed streams are then passed inindirect heat exchange relationship to cool the compressed stream. Thecooled stream is then passed to a separation zone to separate the morevolatile low molecular weight components from the less volatile highermolecular weight components.

Thus, an object of this invention is to-provide a method for theseparation of higher molecular weight components from lower molecularweight components contained in vaporous fluids. Another object of thisinvention is to provide a method for separating ethane and highermolecular weight components of a natural gas stream from methane andother components more volatile than said ethane, with a minimum amountof equipment, plant investment, and operating costs.

Other aspects, objects, and advantages of the invention will be apparentto those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided a process forcryogenically separating higher molecular weight components of avaporous fluid feed stream from lower molecular weight components ofsaid feed stream, which process comprises: (a) expanding a firstvaporous fluid feed stream and decreasing the temperature and pressurethereof; (b) compressing a second vaporous fluid feed stream containingsaid higher and said lower molecular weight components and increasingthe temperature and pressure thereof; ('c) passing said compressedsecond feed stream in indirect heat exchange relationship with saidexpanded first feed stream; (d) passing said second feed stream fromsaid heat exchange to a first separation zone; (e) withdraw ing anessentially vaporous stream comprising said lower molecular weightcomponents from said first separation zone as a product of the process;and (f) withdrawing an essentially liquid stream comprising said highermolecular weight components from said'first separation zone.

A number of advantages are obtained or realized in the practice of theinvention. The process of the invention provides an economical methodfor separating higher molecular weight components from lower molecularweight components of a vaporous fluid utilizing the minimum amount ofconventional equipment, plant investment, and lower operating costs. Ina preferred embodiment the energy contained in a relatively highpressure feed stream, which is preferably lean in the higher molecularweight components, is utilized to compress a relatively low pressurefeed stream which is preferably rich in said higher molecular weightcomponents. In the practice of the invention, the valuable highermolecular weight components are recovered in liquid form which isconvenient for storage and subsequent use in the manufacture ofpetrochemicals. The stream of lower molecular weight components which isrecovered in the practice of the invention is more suited for its mostpractical use, e.g., a high B.t.u./cu.. ft. fuel gas, and thus its valueis also increased.

FIG. 1 is a diagrammatic flow sheet illustrating one embodiment of theinvention.

FIG. 2 is a diagrammatic flow sheet illustrating another presently morepreferred embodiment of the invention.

Referring now to the drawings, wherein likereference numerals have beenemployed to denote like elements, the invention will be more fullyexplained. It will be understood that many valves, pumps, controlinstruments, and other conventional equipment not necessary forexplaining theinvention have been omitted for the sake of brevity. Forconvenience, and not by way of limitation, the invention is illustratedwith reference to the separation of ethane and higher molecular weightcomponents from methane and other components more volatile than saidethane contained in a vaporous fluid feed stream comprising natural gas.The ethane and heavier components of natural gas are oftentimes referredto as natural gas liquids. These liquids" include hydrocarbons such asethane, propane, butanes, pentanes, and some higher molecular weightcomponents, which are valuable as raw materials for preparing variouspetrochemicals. The more volatile" components referred to above includesuch materials as hydrogen, nitrogen, helium, carbon dioxide, and thelike. As used herein and in the claims, unless otherwise specified, theterm lower molecular weight components will be understood to includesuch materials as said more volatile components mentioned above. Itshould also be understood that it is within the scope of the inventionto separate other fractions of natural gas using the methods of theinvention, for example, such as separating ethane and lighter componentsfrom propane and heavier components, or separating propane and lightercomponents from butane and heavier components, etc.

It should also be understood that the representative temperatures andpressures set forth hereinafter in the description of the drawings areonly illustrations of temperatures and pressures which can be utilized.The particular temperatures and pressures utilized in, any particularseparation will be dependent upon the nature and composition of thevaporous fluid feed stream, upon the particular heat exchange surfacearea available, and upon the initial temperature and pressure of thevaporous fluid feed streams.

As illustrated in FIG. 1, a stream of natural gas is withdrawn fromconduit and passed via conduit 12 into dehydrator l4. Said conduit 10can be a pipe line or a conduit from a well head. The natural gas inconduit 12 can be at a temperature of about 90 F. and is preferablyunder a relatively high pressure of about 605 psia. Said dehydrator 14can comprise any suitable means for the dehydration of a natural gas.Such dehydration means and processes are well known in the art and neednot be described in detail herein. If dehydration of the gas is notnecessary, dehydrator 14 can be bypassed by means not shown. The naturalgas is passed from dehydrator 14 via conduit 16, through heat exchanger18, and introduced at a temperature of about 14" F. and a pressure ofabout 600 psia into the expander section of an expander-compressor unit.

A second stream of natural gas is withdrawn from conduit 22 via conduit24 and passed, if necessary or desirable, through dehydrator 26. Conduit22 can be a pipe line a conduit from a well head. The natural gas inconduit 24 can be at a temperature of about 90 F. and is preferablyunder a relatively low pressure of about 100 psia. Preferably, saidsecond stream of natural gas will be rich in higher molecular weightcomponents such as ethane and heavier materials.The natural gas fromdehydrator 26 is passed via conduit 28 into the compressor section 30 ofsaid expandercompressor unit.

Said first stream of natural gas is expanded in expander section 20whereby the temperature and pressure thereof are decreased. The energyreleased upon said expansion is utilized to drive compressor section 30.Said second stream of natural gas is compressed in compressor section 30whereby the temperature and pressure thereof are increased. The effluentfrom said compressor section 30 is passed through heat exchanger 29 forcooling. The expanded first natural gas stream is withdrawn fromexpander section 20 via conduit 32 at a temperature of about l24 F. anda pressure of about 150 psia, and passed in heat exchange relationshipin heat exchanger 34 with the compressed second stream of natural gaswithdrawn from compressor section 30 via conduit 36. The compressed andnow cooled second stream of natural gas is passed from heat exchanger 34via conduit 38 at a temperature of about 102 F. and a pressure of about135 psia into first separation zone 40. Said separation zone 40 cancomprise any suitable separation means such as a liquid-vapor separatortower. An essentially vaporous stream comprising said lower molecularweight components, and containing only small amounts of ethane andheavier components, is withdrawn from said first separation zone 40 viaconduit 42 at a temperature of about 102 F. and a pressure of about 135psia as a product of the process. Preferably, at least a portion of thestream in conduit 42 is through heat exchanger 29 via conduits 25 and27. Said product can be passed to storage or other use via conduit 44 orcan be passed via conduit 46 into indirect heat exchange relationshipwith said first stream of natural gas in heat exchanger 18 and thenpassed via conduit 48 to storage or other use. The expanded first streamof natural gas can be passed from heat exchanger 34 via conduit 50 intoconduit 42 to be combined with the stream therein as a part of saidproduct. If desired, said expanded first stream of natural gas can bewithdrawn via conduit 52.

An essentially liquid stream comprising said higher molecular weightcomponents, and containing only small amounts of methane and othercomponents more volatile than ethane, is withdrawn from first separationzone 40 via conduit 54 at a temperature of -lO2 F. and a pressure of 135psia, and if desired can be passed via conduit 56 to storage or otheruse. Preferably, said stream in conduit 54 is passed via conduit 58,controlled by the flow control valve and liquid level control meansshown, through pump 60 wherein the pressure thereon is increased toabout 615 psia, through heater 61, and is then introduced via conduit 62into second liquid-vapor separation zone 64. An essentially vaporousstream comprising lower molecular weight components is withdrawn from 64via conduit 66 at a temperature of 30 F. and a pressure of 610 psia andis preferably passed via conduit 68 into conduit 16 for combining withsaid first stream of natural gas. If desired, the lower molecular weightcomponents in conduit 66 can be passed via conduit 70 to other use. Anessentially liquid stream comprising higher molecular weight componentsis withdrawn from second separation zone 64 via conduit 72 at atemperature of 30 F. and a pressure of 610 psia, controlled by the flowcontrol valve and liquid level control means shown, and passed tostorage or other use.

FIG. 2 illustrates one presently preferred embodiment of the invention.A first feed stream of relatively high pressure natural gas, preferablylean in said high molecular weight components, at a temperature of aboutF. and preferably under a relatively high pressure of about 605 psia, ispassed via conduit 16 through heat exchanger 18, exits therefrom at atemperature of about 14 F. and a pressure of about 600 psia, and ispassed via conduit 19 into expander section 20 of an expander-compressorunit. The expanded gas is withdrawn via conduit 32 at a temperature ofabout l24 F. and a pressure of about 150 psia and passed through heatexchanger 34, exiting therefrom at a temperature of about 40 F. and apressure of about psia. The gas stream from heat exchanger 34 is passedvia conduit 51 into trap or knockout drum 74 for the removal of anyhigher molecular weight components which may have been condensed by thedescribed expansion and cooling steps. Said higher molecular weightcomponents can be withdrawn via conduit 76.

If desired, said stream in conduit 76 can be introduced into conduit 93for introduction into first separation zone 40. Said trap or knockoutdrum 74 may not be necessary or desirable. If so, the stream of gas inconduit 52 can bypass trap 74 via conduit 78 and be introduced intoconduit 80 for removal from the system. Since the gas feed stream inconduit 16 is preferably lean in higher molecular weight components, thestream in conduit 80 will be comprised predominantly of lower molecularweight components. Said stream can be removed via conduit 82 forutilization as fuel gas or other use. Preferably, however, the stream ofgas in conduit 80 is passed via conduit 84 into said heat exchanger 18for indirect heat exchange with the incoming stream of gas in conduit16. Typically, said stream of gas from conduit 84 will exit from heatexchanger 18 via conduit 86 at a temperature of about 60 F. and apressure of about 130 psia.

A second feed stream of natural gas containing higher molecular weightcomponents and lower molecular weight components, and preferably rich insaid higher molecular weight components, is introduced via conduit 28 ata temperature of about 90 F. and a pressure of about I00 psia into thecompressor section 30 of said expander-compressor unit. Saidexpandercompressor unit is a conventional piece of equipment with thecompressor section being driven by the expander or turbine section andthus utilizing the energy contained in the stream of high pressure gasintroduced into the expander section via conduit 19. The compressed andheated gas from expander section is withdrawn via conduit 36, passedthrough heat exchanger 88 from which it exits at a temperature of about20 F. and a pressure of about 140 psia, then passed via conduit 89 intoheat exchanger 90 from which it exits via conduit 91 at a temperature ofabout -3 3 F. and a pressure of about 138 psia, then passed through heatexchanger 92 from which it exits via conduit 93 at a temperature ofabout -l02 F., and is then introduced into first separation 40. Saidseparation zone 40 can comprise any suitable separation means such as aliquid-vapor separator vessel. Preferably, a portion, approximately 50per cent, of the stream in conduit 36 is passed via conduit 31 into heatexchanger 33 and then passed via conduit 35 into conduit 89 forrecombining with the portion of said stream which was passed throughheat exchanger 88.

The separation in separation zone favors the retention of ethane andhigher molecular weight components in the bottoms product describedhereinafter. An essentially vaporous stream comprising lower molecularweight components such as methane and other components more volatilethan ethane, but usually containing a small amount of ethane, iswithdrawn from separation zone 40 via conduit 95 at a temperature ofabout l02 F. and a pressure of about 135 psia, passed through said heatexchanger 90 from which it exits at a temperature of about 30 F. and apressure of about I30 psia, then passed via conduit 96 into said heatexchanger 88 from which it exits via conduit 97 at a temperature ofabout 83 F. and a pressure of about I25 psia. If desired, said stream ofgas in conduit 97 can be combined with the stream of gas in conduit 86.

An essentially liquid stream comprising higher molecular weightcomponents such as ethane and heavier hydrocarbons is withdrawn fromseparation zone 40 via conduit 98, passed through pump 60 from which itexits at a temperature of about-102 F. and a pressure of about 615 psia,then passed via conduit 100 into said heat exchanger 90 from which itexits via conduit 10] at a temperature of about -30 F. and is thenintroduced into second separation zone 64.

Said second separation zone can comprise any suit able separation meanssuch as a liquid-vapor separator vessel. The separation in separationzone 64 favors the retention of ethane and higher molecular weightcomponents in the bottoms product therefrom. An essentially vaporousstream comprising lower molecular weight components is withdrawn fromsaid second separation zone 64 via conduit 66 at a temperature of about-30 F. and a pressure of about 610 psia. Said stream of gas in conduit66 can be withdrawn from the system via conduit 70 for use as fuel gasor other use if desired. Preferably, said stream of gas in conduit 66 ispassed via conduit 68 into said conduit 16 for combining with theincoming stream of gas therein and recycle through the system.

An essentially liquid stream comprising higher molecular weightcomponents is withdrawn from separation zone 64 via conduit 72 by meansof pump 73 and introduced into conduit 75 at a pressure, e.g., about1,505 psia, and a temperature of about 30 F. Said stream in conduit 75is then passed through said heat exchanger 33 from which it exits viaconduit 77 at a temperature of about 83 F. and a pressure, e.g., about1,500 psia. Said stream in conduit 77 comprises the natural gas liquidsproduct of the process.

The individual elements of apparatus employed in the systems illustratedin FIGS. 1 and 2 are conventional apparatus. For example, said heatexchangers can be any suitable type of heat exchanger. Preferably, saidexchangers 18, 34, 88, and are plate-type exchangers.

The following illustrative example will serve to further illustrate theinvention.

EXAMPLE In this illustrative example a first stream of natural gas undera relatively high pressure of about 605 psia, and lean in highermolecular weight components, is introduced via conduit 16 and utilizedessentially as described above in connection with FIG. 2. A secondstream of natural gas under a relatively low pressure of about psia, andrich in higher molecular weight components, is introduced into thesystem via conduit 28 and processed essentially as described above inconnection with FIG. 2. The material balance set forth in Table l belowshows the composition of the principal streams in a system operatedessentially in accordance with FIG. 2 as described above. I

Wists Stream number 16 28 97 68 86 77 Components, mol. percent:

a Million per day. b Gallons per day.

The above example illustrates that the processes of the invention can beutilized to efficiently separate higher molecular weight components fromlower molecular weight components of a vaporous fluid feed stream.

While certain embodiments of the invention have been described forillustrative purposes, the invention is not limited thereto. Variousother modifications or embodiments of the invention will be apparent tothose skilled in the art in view of this disclosure. Such modificationsor embodiments are within the spirit and scope of the disclosure.

We claim:

1. A process for cryogenically separating ethane and higher molecularweight components of a vaporous fluid feed natural gas stream frommethane and components of lower molecular weight and more volatile thanethane in said feed stream, which process comprises:

a. expanding a first vaporous fluid feed natural gas stream lean inethane and said higher molecular weight components and decreasing thetemperature and pressure thereof;

b. compressing a second vaporous fluid feed natural gas stream richer inethane and said higher molecular weight components than said first feedstream and which stream also contains methane and said lower molecularweight components and increasing the temperature and pressure thereof;

0. passing said compressed second feed stream in indirect heat exchangerelationship with said expanded first feed stream;

d. passing said second feed stream from said heat exchange to a firstseparation zone;

e. withdrawing an essentially vaporous stream comprising methane andsaid lower molecular weight components from said first separation zoneand combining therewith said expanded first feed stream obtained in step(c) as a product of the process;

f. withdrawing an essentially liquid stream comprising ethane and saidhigher molecular weight components from said first separation zone g.increasing the pressure of and heating said essentially liquid streamfrom step (f) and passing same to a second separation zone;

h. withdrawing an essentially vaporous stream comprising said lowermolecular weight components from said second separation zone andcombining same with said first teed stream prior to said expansion instep (a); and

. withdrawing an essentially liquid stream comprising said highermolecular weight components from said second separation zone as anotherproduct of the process.

2. A process according to claim 1 wherein: said second feed stream iscompressed in the compressor section of a compressor-expander; saidfirst feed stream is expanded in said expander section; and saidexpander section drives said compressor section.

3. A process according to claim 1 wherein: prior to said step (c), saidcompressed second feed stream is passed in heat exchange relation withsaid product stream from step (e).

4. A process according to claim 3 wherein: said compressed second feedstream is also passed in heat exchange relation with said essentiallyliquid stream from step (g) prior to said step (c).

5. A process according to claim 4 wherein: at least a portion of saidcompressed second feed stream is passed in heat exchange relation withsaid product stream from step (i) prior to said step (c) and also priorto being passed in heat exchange relation with said essentially liquidstream from step (g).

6. A process according to claim 5 wherein: after said step (c), saidexpanded first feed stream is passed in heat exchange relation with saidfirst feed stream prior to the expansion thereof in said step (a).

7. A process according to claim 6 wherein: said expanded first feedstream, after being passed in said heat exchange relation, is combinedwith said product stream from step (e) after said product stream hasbeen passed in heat exchange relation with said compressed second feedstream; and said vaporous stream from step (h) is combined with saidfirst feed stream prior to the expansion thereof in step (a).

8. A process according to claim 7 wherein: said first feed stream andsaid second feed stream each comprises a stream of natural gas; saidhigher molecular weight components comprise ethane and hydrocarbonshaving a molecular weight greater than ethane; and said lower molecularweight components comprise methane and other components more volatilethan ethane.

9. A process according to claim 4 wherein: heat exchange of saidcompressed second feed stream comprises at least two stages prior tosaid step (c), the first stage comprising indirect heat exchange betweensaid compressed second feed stream and said product stream from step (e)to partially cool said compressed second feed stream and the secondstage comprising indirect heat exchange between the two streams in thefirst stage plus said essentially liquid stream from step (g) to furthercool said compressed second feed stream which is then passed to saidstep (c) for further cooling by indirect heat exchange with saidexpanded first feed stream.

2. A process according to claim 1 wherein: said second feed stream iscompressed in the compressor section of a compressor-expander; saidfirst feed stream is expanded in said expander section; and saidexpander section drives said compressor section.
 3. A process accordingto claim 1 wherein: prior to said step (c), said compressed second feedstream is passed in heat exchange relation with said product stream fromstep (e).
 4. A process according to claim 3 wherein: said compressedsecond feed stream is also passed in heat exchange relation with saidessentially liquid stream from step (g) prior to said step (c).
 5. Aprocess according to claim 4 wherein: at least a portion of saidcompressed second feed stream is passed in heat exchange relation withsaid product stream from step (i) prior to said step (c) and also priorto being passed in heat exchange relation with said essentially liquidstream from step (g).
 6. A process according to claim 5 wherein: aftersaid step (c), said expanded first feed stream is passed in heatexchange relation with said first feed stream prior to the expansionthereof in said step (a).
 7. A process according to claim 6 wherein:said expanded first feed stream, after being passed in said heatexchange relation, is combined with said product stream from step (e)after said product stream has been passed in heat exchange relation withsaid compressed second feed stream; and said vaporous stream from step(h) is combined with said first feed stream prior to the expansionthereof in step (a).
 8. A process according to claim 7 wherein: saidfirst feed stream and said second feed stream each comprises a stream ofnatural gas; said higher molecular weight components comprise ethane andhydrocarbons having a molecular weight greater than ethane; and saidlower molecular weight components comprise methane and other componentsmore volatile than ethane.
 9. A process according to claim 4 wherein:heat exchange of said compressed second feed stream comprises at leasttwo stages prior to said step (c), the first stage comprising indirectheat exchange between said compressed second feed stream and saidproduct stream from step (e) to partially cool said compressed secondfeed stream and the second stage comprising indirect heat exchangebetween the two streams in the first stage plus said essentially liquidstream from step (g) to further cool said compressed second feed streamwhich is then passed to said step (c) for further cooling by indirectheat exchange with said expanded first feed stream.